There is a general need for measuring output quality of a printing system. Results from such quality measurement may be used to fine-tune and configure parameters of the printing system for improved performance. Traditionally, quality measurement has been performed in an offline fashion through manual inspection of an output print from the printing system.
With ever increasing printing speeds and volumes, the need for automated real-time detection of print defects to maintain print quality has increased. Timely identification of print defects can allow virtually immediate corrective action, such as re-printing, to be taken, which in turn reduces waste in paper and ink or toner, while improving efficiency. Some detection systems can operate independent of user or operator input, and may be considered automatic detection systems.
A number of automatic print defect detection systems have been developed. In some arrangements, the systems involve the use of an image acquisition device, such as a CCD (charge-coupled device) camera, to capture or scan an image of a document printout (also referred to as an output print). The scan image is then compared to an image (referred to as the original image) of an original source input document. Discrepancies identified during the comparison can be flagged as print defects.
Real-time print defect detection for high volume printing is an exceptionally challenging problem because defect detection requires very accurate image alignment between the scan image and the original image, and a very high processing speed. Some existing print defect detection methods are unsuitable for real-time processing because the methods are too slow. Other methods are insufficiently accurate and are sensitive to noise and distortion. The noise and distortion either results in a high rate of false defect detections, or a coarse detection method, where only large defects are detected.